The present invention relates generally to an air-core magnetic flux guide.
In free space, flux lines from a magnet (either permanent magnet or electromagnet) diverge after leaving a pole, as shown in FIG. 1. The magnet must therefore be located close to a target volume (a volume of space through which passage of the magnetic flux is desired) if significant flux is to pass through this volume.
It may be desirable to locate the magnet away from the target volume. Currently, a solid rod of ferromagnetic (high permeability) material may be used to guide flux to a target volume remote from the magnet, as in FIG. 2a. Another configuration is shown in FIG. 2b. The high permeability of the ferromagnetic material compared to free space provides a low reluctance path for the magnetic flux. The flux therefore tends to follow this path, in order to bring the system to the lowest energy state. A percentage of flux will take a free space path as leakage flux. There will also be fringing flux at sharp edges and other discontinuities.
Flux may also be concentrated by using a high permeability material. FIG. 2c illustrates this concept to demonstrate the principles involved. In accordance with Ampere's Law, the flux into a node must be equal to the flux out of that node. A reduction in cross-sectional area of a ferromagnetic medium will cause a proportional increase in magnetic flux density, at low flux density levels. This a commonly practiced technique for designing magnetic pole faces and will work until saturation of the ferromagnetic material is approached (typically around two Tesla).
One disadvantage of using ferromagnetic material in these applications is the large mass of the ferromagnetic flux guide. A further disadvantage is the leakage flux, which can cause interference with other devices and instruments near the magnet and flux guides, and reduces the amount of flux passing through the target volume. A third disadvantage is the fringing effect at the ends of the flux guide, which can interfere with the uniformity of the magnetic field through the target volume. A fourth disadvantage occurs as the magnetic flux density approaches the saturation density of the ferromagnetic material. For iron, this is about two Tesla. As saturation is approached, the material's permeability decreases to the point that it is not much higher than the free space surrounding the material. Consequently, much of the flux is not constrained by the material, and seeks a free-space path as leakage flux.
The following U.S. Patents are of interest.
U.S. Pat. No. 3,317,286--Sorbo PA1 U.S. Pat. No. 3,331,041--Bogner PA1 U.S. Pat. No. 3,378,691--Swartz PA1 U.S. Pat. No. 4,409,579--Clem
In particular, the Sorbo patent teaches a method of making superconductive material. The patent to Bogner teaches a superconductive device for shielding magnetic fields comprising a structure of hard superconductive material and sheet members of good heat conducting metal. The patent to Swartz teaches a method of shielding a low magnetic field from a high magnetic field by providing a plurality of concentric superconductor material around the low magnetic field. The patent to Clem teaches a device which includes a solenoid substantially extensive with and overlying the superconducting cylinder.